Noise level meters are measuring instruments for obtaining important measured values of Environmental Quality Standards for Noise (Environment Agency Notification No. 64 of Sep. 30, 1998), Noise Regulation Law (Law No. 98 of Jun. 10, 1968), and the like. The performance of commonly used noise level meters is prescribed by JIS C 1502, and the frequency range to be measured is from 20 Hz to 8000 Hz. In addition, a sound insulation performance of earplugs which have been conventionally used as a noise prevention measure is from 125 Hz to 8000 Hz (see Patent Document 1), and it has been known that the upper limit of the frequency range in general audibility examinations is 8000 Hz. Therefore, the noise prevention measure has been conventionally targeted to a sound range of 8000 Hz or less.
In addition, in audiometers for audibility diagnosis in conformity with the Japanese Industrial Standard, the highest diagnosis frequency is 8000 Hz, and the high frequency range higher than 8000 Hz is exempt from the diagnosis (see Non-Patent Document 1). Further, also in ear protectors in conformity with the Japanese Industrial Standard, the range of 8000 Hz or less is the targeted frequency of the Standards.
However, noise generated from dental treatment instruments such as an air turbine or a scaler includes a frequency component having a significantly high sound pressure level (see Non-Patent Document 2), and its sound range is higher than 8000 Hz.
In addition, noise generated from the air turbine, the scaler, or the like can cause a painful feeling to a patient. Therefore, in many dental treatment facilities, noise generated from the air turbine, the scaler, or the like is made less audible due to surrounding acoustic effects such as playing music.